The present invention relates generally to a device for measuring the flow rate of intake air flowing into an internal combustion engine, and more particularly a device for measuring the flow rate of intake air flowing into the engine by detecting the degree of opening of a sensing vane disposed within an air intake pipe at the upstream of a throttle valve.
In the devices of the type described above, which have the sensing vane of a butterfly type, the angular displacement of the vane is not in linear proportion to the area of the opening defined between the vane and the inner wall of the air intake pipe, and it is not guaranteed that the pressure difference across the sensing vane will be maintained constant. As a result, the flow rate of intake air is not in linear proportion to the angular displacement of the sensing vane. Therefore when the conventional devices are incorporated in the fuel injection systems of internal combustion engines, the signal representative of the degree of opening of the sensing vane must be so processed as to be in linear relation with the flow rate of intake air because the quantity of fuel to be injected into the cylinders must be metered depending upon the flow rate of intake air so as to provide a predetermined air-fuel ratio. In correcting the signal, not only the area of the opening defined between the sensing vane and the inner wall of the air intake pipe but also the pressure difference across the sensing vane must be taken into consideration. The signal must be corrected based upon the data obtained from the experiments. Therefore the air-fuel ratio cannot be controlled with a desired degree of accuracy. In addition, the conventional devices are complex in construction because various devices must be incorporated in order to correct the signal.